> Tumblr had issues with jailbait and childporn. Even with a lot of moderation and policing, it was difficult to keep under control.

That's because they had crappy design:

* How does one report a post? Click the share button (I'm not clicking something labeled share on child porn).

* Can moderators delete posts and the comment chain that allows for coordination? Nope, just the images.

Doesn't matter how moderation one does if the tools aren't there.